Low levels of amniotic fluid placental alkaline phosphatase in Down's syndrome

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Abstract
To investigate amniotic fluid placental alkaline phosphatase (PLAP) levels in normal and trisomy 21 pregnancies. Cross sectional study. A tertiary referral prenatal diagnostic service. Three hundred and eleven women with singleton pregnancies of normal karyotype between 10 and 23 weeks gestation and 31 women with pregnancies associated with trisomy 21 Down's syndrome. PLAP levels were measured by immunoradiometric assay in amniotic fluid obtained by amniocentesis. Amniotic fluid PLAP was detectable from 12 weeks gestation and the median value rose to a peak of 4.57 iu/l at 18 weeks. Pregnancies associated with Down's syndrome had significantly lower levels with a median multiple of median (MoM) of 0.638, (U = 3374, P = 0.0016, 95% CI = 0.50, 0.89). For the 20 women with trisomy 21 pregnancies detected at 16 to 18 weeks, the median MoM was 0.482, (U = 3694, P = 0.0011, 95% CI = 0.37, 0.85). These data demonstrates that PLAP levels are reduced in the amniotic fluid of women carrying a fetus with trisomy 21.